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Boltar · September 8th 04, 09:24 AM posted to uk.transport.london

(Colin Rosenstiel) wrote in message ...
In article ,
(Neil Williams) wrote:

If the electric motors are of the three-phase type often employed on
recent[1] rail stock, the "gear change" is actually a change in the AC
frequency being applied to the motors as they speed up. It (put
simply) has broadly the same effect as a gear change, but no physical
"gear" is being "changed" as such.

[1] While all EMUs built since the 1990s have this technology, it's
not always audible for various reasons. The Class 323 EMUs used in
the Manchester area seem the most obvious.

ITYF that the noise is more pronounced on older three phase designs.
Current production uses a later three-phase technology which has a less
pronounced "gear change" effect.

The older AC drives used GTO thyristors which operated at a frequency the
human ear can hear and because they had a max operating frequency some
sort of electronic equivalent of gear changing had to occur to let them
drived the motors at the full range of speeds required (don't know the details
I'm not an electronic engineer). The newer drives use IGB transisters which
operate at a much higher frequency though if the new stock on the northern
line is anything to go by you can still hear a very high pitched whine.

B2003